Imaging device and image acquisition device

ABSTRACT

An imaging device including at least one pixel, where each of the at least one pixels includes a photoelectric conversion layer having a first surface and a second surface being on a side opposite to the first surface; a first electrode located on the first surface; a second electrode located on the first surface, the second electrode being separated from the first electrode, a first voltage being applied to the second electrode; a third electrode located on the second surface, the third electrode opposing to the first electrode and the second electrode, a second voltage being applied to the third electrode; and an amplifier transistor having a gate electrically connected to the first electrode, where an absolute value of a difference between the first voltage and the second voltage is larger than an absolute value of a difference between the second voltage and a voltage of the first electrode.

PRIORITY INFORMATION

This is a continuation of U.S. patent application Ser. No. 15/423,397filed Feb. 2, 2017, which is a continuation of U.S. patent applicationSer. No. 14/878,180 filed Oct. 8, 2015, now U.S. Pat. No. 9,602,743,which claims priority of Japanese Patent Application No. 2014-216210filed on Oct. 23, 2014. The entire disclosures of the above-identifiedapplications, including the specifications, drawings and claims areincorporated herein by reference in their entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to an imaging device and an imageacquisition device.

2. Description of the Related Art

A lamination type imaging device is proposed as an imaging device of themetal oxide semiconductor (MOS) type. In a lamination type imagingdevice, photoelectric conversion films are laminated on an outermostsurface of a semiconductor substrate. In the photoelectric conversionfilms, charges are generated through photoelectric conversion. Thegenerated charges are accumulated in a charge accumulation region. Theaccumulated charges are read by a charge coupled device (CCD) circuit ora complementary MOS (CMOS) circuit in the semiconductor substrate.

In a typical lamination type imaging device, a photoelectric conversionfilm is integrally formed throughout a plurality of unit pixel cells andthus, the photoelectric conversion film is not compartmented for everyunit pixel cell. Therefore, lamination type imaging devices haveproblems of carrier crosstalk and color crosstalk between pixels causedby inflow of a signal charge of an adjacent pixel, for example. To solvethese problems, Japanese Unexamined Patent Application Publication No.2008-112907 discloses that a shield electrode is provided in a manner tosurround a pixel electrode and the shield electrode is connected toground.

SUMMARY

As described above, further suppression of crosstalk is required inlamination type imaging devices.

In one general aspect, the techniques disclosed here feature an imagingdevice, comprising: at least one pixel, where each of the at least onepixels includes a photoelectric conversion layer having a first surfaceand a second surface being on a side opposite to the first surface; afirst electrode located on the first surface; a second electrode locatedon the first surface, the second electrode being separated from thefirst electrode, a first voltage being applied to the second electrode;a third electrode located on the second surface, the third electrodeopposing to the first electrode and the second electrode, a secondvoltage being applied to the third electrode; and an amplifiertransistor having a gate electrically connected to the first electrode,where an absolute value of a difference between the first voltage andthe second voltage is larger than an absolute value of a differencebetween the second voltage and a voltage of the first electrode.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a schematic view illustrating the circuitconfiguration of an imaging device according to a first embodiment;

FIG. 2 is an example of a schematic sectional view illustrating a unitpixel cell in the imaging device according to the first embodiment;

FIG. 3 is an example of a schematic plan view illustrating a pixelelectrode and a shield electrode;

FIG. 4A is an example of a schematic sectional view illustrating acharge capture region which is formed in a photoelectric conversionlayer in a case where a shield voltage V1 is applied to the shieldelectrode;

FIG. 4B is an example of a schematic plan view illustrating the chargecapture region which is formed in the photoelectric conversion layer ina case where the shield voltage V1 is applied to the shield electrode;

FIG. 4C is an example of a schematic sectional view illustrating acharge capture region which is formed in the photoelectric conversionlayer in a case where a shield voltage V2 is applied to the shieldelectrode;

FIG. 4D is an example of a schematic plan view illustrating the chargecapture region which is formed in the photoelectric conversion layer ina case where the shield voltage V2 is applied to the shield electrode;

FIG. 5 is an example of a graph illustrating a relation between a shieldvoltage and a sensitivity output;

FIG. 6A is a block diagram illustrating the configuration of an imageacquisition device according to a second embodiment;

FIG. 6B is an example of a schematic view illustrating the briefconfiguration of an illumination system in the image acquisition deviceaccording to the second embodiment;

FIG. 7A is an example of a schematic view illustrating a process foracquiring an image by the image acquisition device in a case where theshield voltage V1 is applied to the shield electrode;

FIG. 7B is an example of a schematic view illustrating a process foracquiring an image by the image acquisition device in a case where theshield voltage V1 is applied to the shield electrode;

FIG. 8 is an example of a schematic view illustrating a pixelarrangement of an image in a case where the shield voltage V1 is appliedto the shield electrode;

FIG. 9A is an example of a schematic view illustrating a pixelarrangement of an image in a case where the shield voltage V2 is appliedto the shield electrode;

FIG. 9B is an example of a schematic view illustrating a pixelarrangement of an image in a case where the shield voltage V2 is appliedto the shield electrode;

FIG. 9C is an example of a schematic view illustrating a pixelarrangement of an image in a case where the shield voltage V2 is appliedto the shield electrode;

FIG. 10 is an example of a schematic view illustrating a pixelarrangement of an image in a case where the shield voltage V2 is appliedto the shield electrode; and

FIG. 11 is another example of a schematic view illustrating the overviewconfiguration of the illumination system in the image acquisition deviceaccording to the second embodiment.

DETAILED DESCRIPTION

Embodiments according to the present disclosure will be described belowwith reference to the accompanied drawings. In the followingembodiments, an example in which in a pair of a hole and an electronwhich is generated through photoelectric conversion, a hole is detectedas a signal charge will be described. A signal charge may be anelectron. Here, the present disclosure is not limited to the followingembodiments. An arbitrary alteration can be made within a scope of theeffect of the present disclosure. Further, one embodiment may becombined with another embodiment. In the following description,constituent elements which are identical or similar to each other aregiven identical reference characters. Further, duplicate description issometimes omitted.

First Embodiment

An imaging device according to a present embodiment is described withreference to FIGS. 1 to 5.

(Configuration of Imaging Device 101)

FIG. 1 schematically illustrates the circuit configuration of an imagingdevice 101 according to a first embodiment. The imaging device 101includes a plurality of unit pixel cells 14 and peripheral circuits.

The unit pixel cells 14 are arranged two-dimensionally, that is,arranged in a row direction and a column direction so as to form aphotosensitive region (pixel region) on a semiconductor substrate. Here,arrangement of the unit pixel cells 14 is not limited to a lattice-likeshape, but the unit pixel cells 14 may be arranged in a honeycomb shape,for example. Further, the imaging device 101 may be a line sensor. Inthis case, the unit pixel cells 14 may be arranged one-dimensionally. Inthis specification, the row direction and the column direction representdirections in which a row and a column extend respectively. In thepresent embodiment, a vertical direction is the column direction and ahorizontal direction is the row direction.

Each of the unit pixel cells 14 includes a photoelectric conversion unit10, an amplifier transistor 11, a reset transistor 12, and an addresstransistor (row selection transistor) 13. In the present embodiment, thephotoelectric conversion unit 10 includes a pixel electrode 50 and ashield electrode 61. To the shield electrode 61, a shield voltage isapplied. A shield voltage is lower than an initialization voltage whichresets the pixel electrode 50. The shield voltage is preferably anegative voltage. Accordingly, crosstalk among pixels can be furthersuppressed. Details will be described later.

The imaging device 101 includes a shield voltage generation circuit 60so as to apply a shield voltage to the shield electrode 61. The shieldvoltage generation circuit 60 is provided in an outside of thephotosensitive region as a part of the peripheral circuits.

The pixel electrode 50 is connected to a gate electrode of the amplifiertransistor 11. Signal charges collected by the pixel electrode 50 areaccumulated in a charge accumulation node 24 which is positioned betweenthe pixel electrode 50 and the gate electrode of the amplifiertransistor 11. In the present embodiment, a signal charge is a hole, buta signal charge may be an electron.

A voltage corresponding to the amount of signal charges accumulated inthe charge accumulation node 24 is applied to the gate electrode of theamplifier transistor 11. The amplifier transistor 11 amplifies thisvoltage. The amplified voltage is selectively read by the addresstransistor 13 as a signal voltage. A source or drain electrode of thereset transistor 12 is connected to the pixel electrode 50. The resettransistor 12 resets signal charges accumulated in the chargeaccumulation node 24. In other words, the reset transistor 12 resets apotential of the gate electrode of the amplifier transistor 11 and apotential of the pixel electrode 50 at the predetermined timing, forexample, per frame.

The unit pixel cells 14 selectively perform the above-describedoperation. Therefore, the imaging device 101 includes a power supplywiring 21, a vertical signal line 17, an address signal line 26, and areset signal line 27. Each of these lines is connected to the unit pixelcells 14. In particular, the power supply wiring 21 is connected to asource or drain electrode of the amplifier transistor 11. The verticalsignal line 17 is connected to a source or drain electrode of theaddress transistor 13. The address signal line 26 is connected to a gateelectrode of the address transistor 13. The reset signal line 27 isconnected to a gate electrode of the reset transistor 12.

Further, the imaging device 101 includes a photoelectric conversion unitcontrol line 16. Identical constant voltages are respectively applied toall photoelectric conversion units 10 of the unit pixel cells 14 via thephotoelectric conversion unit control line 16.

The imaging device 101 includes a vertical scanning circuit 15, ahorizontal signal read circuit 20, a plurality of column signalprocessing circuits 19, a plurality of load circuits 18, and a pluralityof differential amplifiers 22 as peripheral circuits. The verticalscanning circuit 15 is also referred to as a row scanning circuit. Thehorizontal signal read circuit 20 is also referred to as a columnscanning circuit. The column signal processing circuit 19 is alsoreferred to as a row signal accumulation circuit. The differentialamplifier 22 is also referred to as a feedback amplifier.

The vertical scanning circuit 15 is connected to the address signal line26 and the reset signal line 27. The vertical scanning circuit 15selects a plurality of unit pixel cells 14 in a row unit so as to readsignal voltages and reset potentials of the pixel electrodes 50. Thepower supply wiring (source follower power supply) 21 supplies apredetermined power supply voltage to each of the unit pixel cells 14.The horizontal signal read circuit 20 is electrically connected to thecolumn signal processing circuits 19. The column signal processingcircuit 19 is electrically connected to a plurality of unit pixel cells14 which are disposed on each column via the vertical signal line 17corresponding to each column. The load circuit 18 is electricallyconnected to each vertical signal line 17. The load circuit 18 and theamplifier transistor 11 form a source follower circuit.

The differential amplifiers 22 are provided to correspond to respectivecolumns. An input terminal on a negative side of the differentialamplifier 22 is connected to a corresponding vertical signal line 17.Further, an output terminal of the differential amplifier 22 isconnected to the unit pixel cells 14 via a feedback line 23 whichcorresponds to each column.

The vertical scanning circuit 15 applies a row selection signal forcontrolling on/off of the address transistor 13 to the gate electrode ofthe address transistor 13 via the address signal line 26. Accordingly, arow of the unit pixel cells 14 which are reading objects is selected.From the unit pixel cells 14 of the selected row, signal voltages areread on the vertical signal line 17. Further, the vertical scanningcircuit 15 applies a reset signal for controlling on/off of the resettransistor 12 to the gate electrode of the reset transistor 12 via thereset signal line 27. Accordingly, a row of the unit pixel cells 14which are objects of a reset operation is selected. The vertical signalline 17 transfers the signal voltages which are read from the unit pixelcells 14 selected by the vertical scanning circuit 15 to the columnsignal processing circuit 19.

The column signal processing circuit 19 performs noise suppressionsignal processing typified by correlation double sampling andanalog-digital conversion (AD conversion), for example.

The horizontal signal read circuit 20 sequentially reads signals from aplurality of column signal processing circuits 19 and outputs signals toa horizontal common signal line (not illustrated).

The differential amplifier 22 is connected to the drain or sourceelectrode of the reset transistor 12 via the feedback line 23.Accordingly, the differential amplifier 22 receives an output value ofthe address transistor 13 on a negative terminal thereof when theaddress transistor 13 and the reset transistor 12 are in a conductivestate. The differential amplifier 22 performs a feedback operation so asto set a gate potential of the amplifier transistor 11 to apredetermined feedback voltage. At this time, an output voltage value ofthe differential amplifier 22 is a positive voltage which is 0 V orapproximately 0 V. A feedback voltage represents an output voltage ofthe differential amplifier 22 and is an initialization voltage forresetting signal charges which are accumulated in the gate electrode ofthe amplifier transistor 11, the pixel electrode 50, and the like.

(Device Configuration of Unit Pixel Cell 14)

FIG. 2 schematically illustrates a cross section of the deviceconfiguration of the unit pixel cell 14.

The unit pixel cell 14 includes a semiconductor substrate 31, a chargedetection circuit 25, and the photoelectric conversion unit 10. Thesemiconductor substrate 31 is a p-type silicon substrate, for example.The charge detection circuit 25 detects a signal charge which iscaptured by the pixel electrode 50 and outputs a signal voltage. Thecharge detection circuit 25 includes the amplifier transistor 11, thereset transistor 12, and the address transistor 13. The charge detectioncircuit 25 is formed on the semiconductor substrate 31.

The amplifier transistor 11 includes n-type impurity regions 41C and 41Dwhich are formed in the semiconductor substrate 31 and serve as a drainelectrode and a source electrode respectively. The amplifier transistor11 further includes a gate insulation layer 38B which is positioned onthe semiconductor substrate 31 and a gate electrode 39B which ispositioned on the gate insulation layer 38B.

The reset transistor 12 includes n-type impurity regions 41B and 41Awhich are formed in the semiconductor substrate 31 and serve as a drainelectrode and a source electrode respectively. The reset transistor 12further includes a gate insulation layer 38A which is positioned on thesemiconductor substrate 31 and a gate electrode 39A which is positionedon the gate insulation layer 38A.

The address transistor 13 includes n-type impurity regions 41D and 41Ewhich are formed in the semiconductor substrate 31 and serve as a drainelectrode and a source electrode respectively. The address transistor 13further includes a gate insulation layer 38C which is positioned on thesemiconductor substrate 31 and a gate electrode 39C which is positionedon the gate insulation layer 38C. The n-type impurity region 41D isshared by the amplifier transistor 11 and the address transistor 13.Accordingly, the amplifier transistor 11 and the address transistor 13are connected in series.

Between adjacent unit pixel cells 14 and between the amplifiertransistor 11 and the reset transistor 12 on the semiconductor substrate31, an element isolation region 42 is provided. Adjacent unit pixelcells 14 are electrically isolated from each other by the elementisolation region 42. Further, a leak of signal charges which areaccumulated in the charge accumulation node is suppressed.

On a surface of the semiconductor substrate 31, interlayer insulationlayers 43A, 43B, and 43C are laminated. In the interlayer insulationlayer 43A, a contact plug 45A, a contact plug 45B, and a wiring 46A areembedded. The contact plug 45A is connected to the n-type impurityregion 41B of the reset transistor 12. The contact plug 45B is connectedto the gate electrode 39B of the amplifier transistor 11. The wiring 46Aconnects the contact plug 45A and the contact plug 45B. Accordingly, then-type impurity region 41B (drain electrode) of the reset transistor 12is electrically connected to the gate electrode 39B of the amplifiertransistor 11.

The photoelectric conversion unit 10 is provided on the interlayerinsulation layer 43C. The photoelectric conversion unit 10 includes anupper electrode 52, a photoelectric conversion layer 51, the pixelelectrode 50, and the shield electrode 61. The photoelectric conversionlayer 51 is interposed between the upper electrode 52 and the pixelelectrode 50 and between the upper electrode 52 and the shield electrode61. The pixel electrode 50 and the shield electrode 61 are provided onthe interlayer insulation layer 43C. The upper electrode 52 is made of aconductive transparent material such as ITO, for example. The pixelelectrode 50 and the shield electrode 61 are made of a metal such asaluminum and copper, polysilicon to which impurity is doped andconductivity is imparted, or the like.

As illustrated in FIG. 2, the unit pixel cell 14 does not have a microlens (on-chip micro lens) on the upper electrode 52 of the photoelectricconversion unit 10. Though not illustrated, the unit pixel cell 14 mayhave a color filter on the upper electrode 52 of the photoelectricconversion unit 10.

FIG. 3 illustrates shapes of the pixel electrode 50 and the shieldelectrode 61 on a surface of the interlayer insulation layer 43C. FIG. 3illustrates the configurations of nine unit pixel cells which areadjacent to each other. As illustrated in FIG. 3, the pixel electrode 50has a rectangular shape in the present embodiment. The shield electrode61 has an opening having a rectangular shape and surrounds the pixelelectrode 50. The pixel electrode 50 and the shield electrode 61 areseparated from each other by a distance L1. The shield electrodes 61 ofrespective unit pixel cells 14 are integrally formed and areelectrically connected to each other.

The pixel electrode 50 may have a circular shape or a polygonal shape.It is preferable that the shield electrode 61 surround the pixelelectrode 50. However, it is enough that the shield electrode 61 ispositioned between the pixel electrodes 50 of two adjacent unit pixelcells 14, and the shield electrode 61 does not have to surround thepixel electrode 50.

In the interlayer insulation layer 43A, a plug 47A is embedded. On theinterlayer insulation layer 43A, a wiring 46B is provided. In theinterlayer insulation layer 43B, a plug 47B is embedded. On theinterlayer insulation layer 43B, a wiring 46C and a wiring 49 areprovided. In the interlayer insulation layer 43C, a plug 47C and a plug48 are embedded. The pixel electrode 50 is connected to the wiring 46Avia the plug 47C, the wiring 46C, the plug 47B, the wiring 46B, and theplug 47A. Further, the shield electrode 61 is connected to the wiring 49via the plug 48. These plugs, contact plugs, and the wirings are made ofa metal such as aluminum and copper, conductive polysilicon to whichimpurity is doped, or the like.

In the present embodiment, the imaging device 101 detects a hole as asignal charge in a pair of a hole and an electron which are generatedthrough photoelectric conversion in the photoelectric conversion layer51. Detected signal charges are accumulated in the charge accumulationnode 24. The charge accumulation node 24 includes the pixel electrode50, the gate electrode 39B, and the n-type impurity region 41B. Thecharge accumulation node 24 further includes the plugs 47A, 47B, and47C, the contact plugs 45A and 45B, and the wirings 46A, 46B, and 46Cwhich connect the pixel electrode 50, the gate electrode 39B, and then-type impurity region 41B with each other.

The photoelectric conversion layer 51 covers the shield electrode 61 andthe pixel electrode 50 on the interlayer insulation layer 43C and iscontinuously formed throughout the whole of a plurality of unit pixelcells 14. The photoelectric conversion layer 51 is made of an organicmaterial or amorphous silicon, for example.

Though not illustrated in FIG. 2, peripheral circuits, specifically, thevertical scanning circuit 15, the horizontal signal read circuit 20, thecolumn signal processing circuits 19, the load circuits 18, and thedifferential amplifiers 22 are also formed on the semiconductorsubstrate 31.

The imaging device 101 can be manufactured by using a commonsemiconductor manufacturing process. Particularly, when a siliconsubstrate is used as the semiconductor substrate 31, the imaging device101 can be manufactured by using various types of silicon semiconductorprocesses.

(Operation of Imaging Device 101)

An operation of the imaging device 101 is now described with referenceto FIG. 1, FIG. 4A, and FIG. 4B.

In a case where a hole is used as a signal charge, a potential of thepixel electrode 50 and a potential of the shield electrode 61 are set tobe lower than a potential of the upper electrode 52. Accordingly, holesgenerated through photoelectric conversion can be gathered toward thepixel electrode 50. In a state in which a voltage of approximately 10 V,for example, is applied to the upper electrode 52, the reset transistor12 is first turned on and then, turned off. Accordingly, a potential ofthe pixel electrode 50 is reset. By this reset, a potential of thecharge accumulation node 24 which includes the pixel electrode 50 is setto an initialization voltage as an initial value, for example, 0 V. Theshield voltage generation circuit 60 generates a shield voltage V1 whichis lower than the initialization voltage, for example, and applies theshield voltage V1 to the shield electrode 61. The shield voltage V1 is−2 V, for example. In the present specification, a voltage applied tothe upper electrode 52 is sometimes referred to as a “counter voltage”.

As illustrated in FIG. 4A, the imaging device 101 does not include amicro lens in each unit pixel cell 14. Therefore, light incident on thephotoelectric conversion unit 10 enters the photoelectric conversionlayer 51 in the unit pixel cell 14 without being condensed. In thephotoelectric conversion layer 51, a pair of a hole and an electron isgenerated through photoelectric conversion. Holes generated through thephotoelectric conversion move to the shield electrode 61 and the pixelelectrode 50. As described above, a voltage lower than that of the pixelelectrode 50 is applied to the shield electrode 61. That is, a potentialdifference between the pixel electrode 50 and the upper electrode 52 issmaller than a potential difference between the shield electrode 61 andthe upper electrode 52. Therefore, generated holes more easily move tothe shield electrode 61 than to the pixel electrode 50. Consequently, asillustrated in FIG. 4A, holes generated in a charge capture region 51Amove to the pixel electrode 50 to be detected as signal charges. Holesgenerated in a region 51B are captured by the shield electrode 61. Thus,light irradiated to the charge capture region 51A in the photoelectricconversion layer 51 is detected as a signal charge. That is, the chargecapture region 51A represents a region which is capable of detectinglight. FIG. 4B is a plan view obtained by viewing the charge captureregion 51A from an upper electrode 52 side. The charge capture region51A has an area which is slightly larger than that of the pixelelectrode 50, for example, on a plane parallel with the photoelectricconversion layer 51.

FIG. 4C is an example of a sectional view schematically illustrating acharge capture region 51A′ formed in a photoelectric conversion layer ina case where a shield voltage V2 which is lower than the shield voltageV1 is applied to the shield electrode 61. FIG. 4D is an example of aplan view schematically illustrating the charge capture region 51A′.When a shield voltage is set lower, the region 51B expands and thecharge capture region 51A′ narrows down.

The shield electrode 61 surrounds the pixel electrode 50 as illustratedin FIG. 3. Therefore, the region 51B in the photoelectric conversionlayer 51 also surrounds the charge capture region 51A. Thus, in theimaging device of the present embodiment, the shield electrode 61actively captures holes around the charge capture region 51A which is apixel region. Therefore, even though holes move from an adjacent unitpixel cell 14, the holes are captured by the shield electrode 61.Accordingly, carrier crosstalk and color crosstalk between adjacentpixels are efficiently suppressed.

The shield electrode 61 captures holes, so that the amount of holescaptured by the pixel electrode 50 is reduced. This represents reductionof the amount of holes which are detected as signal charges amonggenerated holes in each unit pixel cell, that is, represents degradationof sensitivity. However, in the imaging device of the presentembodiment, color mixture and crosstalk between adjacent unit pixelcells are more robustly suppressed by actively discarding a part ofgenerated holes.

In the present embodiment, a shield voltage applied to the shieldelectrode 61 is set lower than an initialization voltage which isapplied to the pixel electrode 50. Accordingly, the region 51B can beenlarged and the charge capture region 51A can be narrowed down. Thatis, a pixel region in the unit pixel cell 14 can be narrowed down. Asdescribed in the following embodiment, the imaging device of the presentembodiment is effective in an aspect in which a small pixel region isdesirable.

Further, a size of the charge capture region 51A can be controlled by apotential difference between an initialization voltage and a shieldvoltage. For example, when a size of the pixel electrode 50 is intendedto be reduced, the size of the pixel electrode 50 varies among aplurality of unit pixel cells 14 due to an error in processing accuracy.However, instead of the reduction of the pixel electrode 50 in size, thesize of the charge capture region 51A can be reduced by a potentialdifference between an initialization voltage and a shield voltage.Accordingly, variation in effective pixel regions for respective pixelscan be suppressed.

Further, an area of the charge capture region 51A can be made smallerthan an area of the pixel electrode 50 by adjusting a potentialdifference between an initialization voltage and a shield voltage.Consequently, an imaging device having a smaller pixel region can berealized.

In an image sensor disclosed in Japanese Unexamined Patent ApplicationPublication No. 2008-112907, a potential barrier is formed between pixelelectrodes by a shield electrode. This potential barrier suppressesmigration of signal charges between pixels to suppress crosstalk betweenthe pixels. Therefore, a voltage applied to a shield voltage varies inJapanese Unexamined Patent Application Publication No. 2008-112907.Further, it is conceivable that crosstalk is suppressed by a methoddifferent from that of the present embodiment, in Japanese UnexaminedPatent Application Publication No. 2008-112907.

FIG. 5 schematically illustrates a relation between a shield voltageapplied to a shield electrode and a sensitivity output of the imagingdevice 101 in a case where a signal charge is a hole. As illustrated inFIG. 5, when a shield voltage is changed, a sensitivity output is alsochanged. When a shield voltage is increased, a sensitivity output isincreased. Accordingly, sensitivity of the imaging device can be changedby setting a value of a shield voltage to an arbitrary value lower thanan initialization voltage.

A signal charge is a hole in the present embodiment, but a signal chargemay be an electron. In this case, a voltage higher than that of theupper electrode 52 is applied to the pixel electrode 50 and the shieldelectrode 61 so as to allow electrons generated through photoelectricconversion to move to the pixel electrode 50 and the shield electrode61. When a signal charge is an electron, a shield voltage is set to behigher than an initialization voltage.

Thus, regardless of whether a signal charge is a hole or an electron,the above-described advantageous effect can be obtained by determining ashield voltage such that an absolute value of a difference between avoltage of the upper electrode and a shield voltage is larger than anabsolute value of a difference between a voltage of the upper electrodeand an initialization voltage.

Second Embodiment

An image acquisition device according to a present embodiment isdescribed below with reference to the accompanying drawings.

In the image acquisition device according to the present embodiment, anobject is brought close to a photoelectric conversion unit of an imagingdevice and light passing through the object is detected by thephotoelectric conversion unit. An irradiation direction of light passingthrough the object is varied to allow an identical pixel to detect lightpassing through different parts of the object. A plurality of imagesignals thus acquired are synthesized to obtain a high resolution image.

FIG. 6A schematically illustrates the configuration of an imageacquisition device 102. The image acquisition device 102 includes anillumination system 81, an imaging device 106, and an image processor90.

As the imaging device 106, the imaging device of the first embodiment isused. FIG. 6B schematically illustrates the configuration of theillumination system 81. The illumination system 81 includes lightsources 81 a to 81 i which are arranged two-dimensionally, for example.

FIG. 7A schematically illustrates the configuration of the illuminationsystem 81 and the configuration around the photoelectric conversion unit10 of the imaging device 106. As illustrated in FIG. 7A, an object 80 isdisposed with a distance L2, for example, from the upper electrode 52 ofthe photoelectric conversion unit 10. The distance L2 is equal to orsmaller than 1 mm. For example, the distance L2 is equal to or largerthan approximately 0.1 μm and equal to or smaller than approximately 10μm. The object 80 is disposed in parallel with the photoelectricconversion unit 10. The imaging device 106 may include an arrangementsurface for holding the object 80. The arrangement surface may be anupper surface of a transparent plate disposed on the upper electrode 52,for example. On the upper electrode 52 of the photoelectric conversionunit 10, a condensing optical element such as a micro lens is notdisposed. The object 80 is a light transmitting cell or ripped tissue,for example, which is held on a prepared slide.

The illumination system 81 is disposed on a position which issufficiently separated from the photoelectric conversion unit 10. FIG.7A shows only the light sources 81 a, 81 b, and 81 c among the lightsources 81 a to 81 i. Among the light sources 81 a, 81 b, and 81 c, thelight source 81 a is disposed near the center of a plurality of unitpixel cells 14, which are arranged two-dimensionally, of the imagingdevice 106. The light sources 81 b and 81 c are disposed away from thevicinity of the center. The light sources 81 a, 81 b, and 81 c arerespectively point light sources and are sufficiently separated from thephotoelectric conversion unit 10. Therefore, illumination light which isparallel light is irradiated to the object 80. As illustrated in FIG.7A, the light source 81 a irradiates the object 80 with illuminationlight from a direction orthogonal to the object 80 disposed above thephotoelectric conversion unit 10. On the other hand, the light source 81b irradiates the object 80 with illumination light from a directionoblique to a normal line of the object 80 as illustrated in FIG. 7B. Thesame goes for the light source 81 c. Thus, the illumination system 81sequentially emits illumination light from a plurality of differentirradiation directions in reference to the object 80 so as to irradiatethe object 80 with the illumination light.

Next, a process for acquiring an image of the object 80 by the imageacquisition device 102 is described.

A predetermined shield voltage V1 which is lower than an initializationvoltage is first applied to the shield electrode 61. As described in thefirst embodiment, signal charges generated in the region 51B move to theshield electrode 61 by a shield voltage. Only signal charges generatedin the charge capture region 51A are detected by the pixel electrode 50.That is, the charge capture region 51A defines a pixel region.

The light source 81 a is first turned on so as to irradiate the object80 with illumination light. The illumination light passing through theobject 80 is incident on the photoelectric conversion unit 10. In lightincident on the photoelectric conversion unit 10, only illuminationlight incident on the charge capture region 51A is detected, asdescribed above. That is, only a region 80A of the object 80 is shot.

Then, as illustrated in FIG. 7B, the light source 81 b is turned on soas to irradiate the object 80 with illumination light. The illuminationlight emitted from the light source 81 b is incident obliquely withrespect to the normal line of the object 80. Therefore, light passingthrough a region 80B, which is on an obliquely upward position withrespect to the charge capture region 51A of the photoelectric conversionlayer 51, is incident on the charge capture region 51A. As can be seenfrom FIG. 7B, illumination light passing through the region 80A of theobject 80 is incident on the region 51B of the photoelectric conversionlayer 51. Therefore, when the light source 81 b is turned on, only theregion 80B of the object 80 is shot.

Subsequently, shooting is performed in the same manner by using thelight source 81 g and the light source 81 h of the illumination system81 illustrated in FIG. 6B. FIG. 8 is a plan view of the object 80 andillustrates regions 80A, 80B, 80G, and 80H shot by using the lightsources 81 a, 81 b, 81 g, and 81 h, respectively. As illustrated in FIG.7B and FIG. 8, the charge capture region 51A of the photoelectricconversion layer 51 is positioned below the region 80A. However, theregions 80B, 80G, and 80H of the object 80 are also detected in thecharge capture region 51A of the photoelectric conversion layer 51 byusing the light sources 81 b, 81 g, and 81 h, respectively. Therefore,all regions of the object 80 are shot through four-time shooting inwhich the light sources 81 a, 81 b, 81 g, and 81 h are used. That is,the charge capture region 51A corresponding to a pixel region can beallowed to detect light passing through different parts of the object80.

The image processor 90 rearranges image signals respectively obtainedthrough shooting using the light sources 81 a, 81 b, 81 g, and 81 h toan arrangement illustrated in FIG. 8 and synthesizes the image signals.That is, the image processor 90 synthesizes the image signals byinterpolating the image signals with each other and generates asynthetic image. Accordingly, a high resolution image of an object,compared to images singly shot by respective light sources 81 a, 81 b,81 g, and 81 h, can be obtained.

A resolving power is determined by a size of the charge capture region51A of the photoelectric conversion layer 51 in the image acquisitiondevice 102. When the size of the charge capture region 51A is reduced, aresolving power with respect to a shot image can be raised. A processfor acquiring an image of the object 80 in a case where the size of thecharge capture region 51A is reduced is described with reference toFIGS. 9A to 9C and FIG. 10.

A predetermined shield voltage V2 is first applied to the shieldelectrode 61. The shield voltage V2 is set such that the size of thecharge capture region 51A of the photoelectric conversion layer 51 isreduced. When a signal charge is a hole, the shield voltage V2 is set tobe lower than an initialization voltage and lower than the shieldvoltage V1 described above. Accordingly, as illustrated in FIG. 9A, thesize of the charge capture region 51A of the photoelectric conversionlayer 51 is smaller than the size of the charge capture region 51Aillustrated in FIGS. 7A and 7B.

The light source 81 a is first turned on so as to irradiate the object80 with illumination light. Accordingly, only the region 80A of theobject 80 is shot.

Then, the light source 81 b is turned on so as to irradiate the object80 with illumination light as illustrated in FIG. 9B. The illuminationlight emitted from the light source 81 b is incident obliquely withrespect to the normal line of the object 80. When the light source 81 bis turned on, only the region 80B of the object 80 is shot.

The light source 81 c is next turned on so as to irradiate the object 80with illumination light as illustrated in FIG. 9C. In a similar manner,the illumination light emitted from the light source 81 c is incidentobliquely with respect to the normal line of the object 80. When thelight source 81 c is turned on, only the region 80C of the object 80 isshot.

Subsequently, shooting is performed in the same manner by using thelight sources 81 d to 81 i of the illumination system 81 illustrated inFIG. 6B. FIG. 10 is a plan view of the object 80 and illustrates regions80A to 801 shot by using the light sources 81 a to 81 i, respectively.The regions 80B to 801 of the object 80 are also detected in the chargecapture region 51A of the photoelectric conversion layer 51 by using thelight sources 81 b to 81 i. Therefore, all regions of the object 80 areshot through nine-time shooting in which the light sources 81 a to 81 iare used. That is, the charge capture region 51A corresponding to eachpixel can be allowed to detect light passing through different parts ofthe object 80.

The image processor 90 rearranges image signals respectively obtainedthrough shooting using the light sources 81 a to 81 i to an arrangementillustrated in FIG. 10 and synthesizes the image signals. Accordingly, ahigh resolution image of an object, compared to images singly shot byusing respective light sources 81 a to 81 i, can be obtained.

Even though a shield voltage is changed, the size of the unit pixel cell14 does not vary and a pixel pitch does not vary either. However, thesize of the charge capture region 51A which is the size of an effectivepixel can be changed. In the image acquisition device 102, the size ofthe charge capture region 51A determines a resolving power. Therefore, ahigh resolution image can be acquired by setting a value of a shieldvoltage such that the size of the charge capture region 51A is reduced.For example, an image can be acquired by one fourth resolving power ofthe unit pixel cell 14 in an example illustrated in FIG. 8. In anexample illustrated in FIGS. 9A to 9C, an image can be acquired by oneninth resolving power of the unit pixel cell 14.

According to the image acquisition device of the present embodiment, thesize of the charge capture region in the photoelectric conversion layercan be changed by changing a shield voltage which is applied to theshield electrode. Accordingly, a resolving power can be changed and ahigh resolution image can be acquired by reducing the size of the chargecapture region.

Here, in the present embodiment, the illumination system 81 includes aplurality of light sources and irradiates the object 80 withillumination light from a plurality of different irradiation directionsin accordance with positions of the light sources. However, theillumination system may include a single light source and a direction ofthe imaging device in which an object is held may be varied. Forexample, the illumination system may be composed of a parallel-lightlight source 81′ and a mechanism 82 which changes a posture of theobject 80, as illustrated in FIG. 11. The mechanism 82 is composed of agoniometer stage 82A and a rotating base 82B, for example. Thegoniometer stage 82A supports the imaging device 106 and the object 80.According to this illumination system, a direction of the object 80 withrespect to the parallel-light light source 81′ can be varied by themechanism 82. Accordingly, the object 80 can receive illumination lightof the parallel-light light source 81′ from a plurality of differentdirections in reference to the object 80.

The imaging device and the image acquisition device according to thepresent disclosure are effective for an image sensor which is used in animaging device typified by a digital camera.

What is claimed is:
 1. An imaging device, comprising: at least one pixel, each of the at least one pixel comprising: a photoelectric conversion layer having a first surface and a second surface being on a side opposite to the first surface; a first electrode located on the first surface; a second electrode located on the first surface, the second electrode being separated from the first electrode, a first voltage being applied to the second electrode; a third electrode located on the second surface, the third electrode opposing to the first electrode and the second electrode, a second voltage being applied to the third electrode; and an amplifier transistor having a gate electrically connected to the first electrode, wherein an absolute value of a difference between the first voltage and the second voltage is larger than an absolute value of a difference between the second voltage and a voltage of the first electrode.
 2. The imaging device according to claim 1, wherein the at least one pixel comprises pixels, and the pixels are arranged one-dimensionally or two-dimensionally.
 3. The imaging device according to claim 1, wherein the voltage of the first electrode is lower than the second voltage, and the first voltage is lower than the voltage of the first electrode.
 4. The imaging device according to claim 3, wherein the first voltage is a negative voltage.
 5. The imaging device according to claim 1, wherein the voltage of the first electrode is higher than the second voltage, and the first voltage is higher than the voltage of the first electrode.
 6. The imaging device according to claim 1, wherein each of the at least one pixel does not comprise a micro lens on a side of the second electrode opposite to the photoelectric conversion layer.
 7. An image acquisition device, comprising: an illumination system that irradiates an object with each of beams in sequence, incident directions of the beams with respect to the object being different from each other; the imaging device according to claim 1 located at a position where the beams passing through the object are inputted, the imaging device acquiring images corresponding to the beams respectively, each of the images having a first resolution; and an image processor that synthesizes the images to generate a synthetic image having a second resolution higher than the first resolution.
 8. The image acquisition device according to claim 7, wherein the imaging device includes an arrangement surface for holding the object, the arrangement surface being located on a side of the second electrode opposite to the photoelectric conversion layer.
 9. The image acquisition device according to claim 7, wherein the image processor synthesizes the images by interpolating the images with each other. 